Wireless communications systems, for example cellular telephony or private mobile radio communications systems, typically provide for radio telecommunication links to be arranged between a plurality of base transceiver stations (BTSs) and a plurality of subscriber units, often termed mobile stations (MSs). The term mobile station generally includes both hand-portable and vehicular mounted radio units.
The communication link from a BTS to an MS is generally referred to as a downlink communication channel. Conversely, the communication link from an MS to a BTS is generally referred to as an up-link communication channel.
Wireless communication systems are distinguished over fixed communications systems, such as the public switched telephone networks (PSTN), principally in that mobile stations move between service providers (and/or different BTS) and in doing so encounter varying radio propagation environments.
In a wireless communication system, each BTS has associated with it a particular geographical coverage area (or cell). The coverage area defines a particular range over which the BTS can maintain acceptable communications with MSs operating in its serving cell. Often these cells combine to produce an expanded system coverage area, with the infrastructure supporting respective cells interconnected via centralised switching equipment.
The coverage area is typically determined by a receiver's ability to receive and decode very low-level signals from the transmitting unit. The range of signal levels that a receiver must be able to receive is termed the ‘dynamic range’ of a radio receiver.
For present day hand-portable or mobile receivers, such as those used in portable two-way radios and cellular phones, a larger dynamic range performance equates to less dropped calls and less cell-to-cell handovers. This further leads to a reduction in system overhead transmissions as well as a general improvement to the system's quality of service. Therefore, it is generally a desirable aim of a receiver designer to increase/improve a receiver's dynamic range.
However, a trade-off exists between dynamic range and current consumption in such receivers. Such a trade-off is particularly important iN the field of portable radio designs, where battery life and therefore power consumption is of critical importance. To obtain a higher dynamic range, the active stages of a receiver's radio frequency (RF) components must run at higher DC currents, thus lowering battery life during standby. Running at higher DC currents consequently causes a reduction in battery life.
One option to compensate for a reduction in battery life, in order to increase a dynamic range, would be to increase the battery capacity. However, such an option is rarely considered as viable in the portable cellular/radio field, as this would require an increase in both the size and the weight of the battery.
In summary, reducing standby time or increasing battery capacity are both highly undesirable for portable products, where low size and weight and long standby times are demanded in order for a product to be competitive in this market.
A radio receiver is often defined in terms of ‘front-end’ and ‘back-end’ characteristics. The front-end of a receiver encompasses all of the RF circuitry whereas the ‘back-end’ encompasses all of the base-band processing circuitry.
In the field of this invention, it is known that most receivers have automatic gain control (AGC), which controls the gain of the final stages of the receiver. Such AGC operation needs to be wide-band when some portion of the AGC circuitry is operational at RF frequencies, or narrow-band if the AGC circuitry is only operational at intermediate or baseband frequencies.
When used in receiver circuits, amplifiers will often encounter a very wide range of signal levels; typically they need to operate (namely be able to receive and recover signals) over a 120-dB range. To prevent overloading of these active components, the receiver's gain must be reduced as the signal strength increases. This is usually achieved automatically by changing the bias point of the RF transistor. However, in addition to the change in gain, the bias changes may also alter the input and output impedances of the receiver's front-end and thereby may make the amplifier operation more nonlinear.
Overall amplifier gain is generally dictated by the performance of two amplifier parameters. The first component is the power gain of the transistor itself and the second component is the loss of gain due to input and output mismatching.
A poor design might then start out with a high-gain mismatched stage. When the bias is changed in order to reduce the gain of the transistor, the input and output impedance matching needs to change in order to provide an improved transfer of power.
A better design would start out with ideal matching when highest gain is required and then benefit from both the mismatch and the transistor gain loss as bias is changed. Such a design would likely require neutralization at the highest gain setting.
It is known that the gain of a transistor is typically changed in one of two ways. A first method is to change the collector current or the collector-to-emitter voltage. Any current changes either above or below the optimum value result in a loss of gain. At lower current levels, the voltage has little effect on the gain, whilst at the higher current levels the collector voltage has a more noticeable effect. The two methods of automatic gain control will then depend on whether the collector current is either increased (forward control) or decreased (reverse control) from the optimum value.
The simplest method of gain control is obtained by reducing the collector current. This will simultaneously decrease the current gain and raise the input impedance. Both effects will decrease the power gain, and the increase in input impedance will result in a further mismatch loss.
In summary, it is known that the provision of stable, AGC circuits in radio products is problematic, complex and typically requires a substantial number of hardware (RF) components. In particular, AGC designs are critical to ensure stability of the RF operation, and to achieve rapid receiver operation, often-termed fast ‘receiver attack times’. The costly hardware needs to be controlled from the base-band processing circuitry, in order to ensure accurate performance, which adds to the complexity of the AGC solution.
A need therefore exists for a high dynamic range radio receiver, particularly for linear technology such as that adopted for TETRA, wherein the above-mentioned disadvantages may be alleviated.